1. The Field of the Invention
The present invention relates generally to thermoplastic films. Specifically, the invention relates to incrementally stretched thermoplastic films with tailored physical properties, and to methods of incrementally stretching thermoplastic films to tailor the physical properties thereof.
2. Background and Relevant Art
Thermoplastic films are a common component in various commercial and consumer products. For example, food wraps, grocery bags, trash bags, sacks, and packaging materials are products that are commonly made from thermoplastic films. Additionally, feminine hygiene products, baby diapers, adult incontinence products, and many other products include thermoplastic films to one extent or another.
Thermoplastic films have a variety of different strength parameters that manufacturers of products incorporating a thermoplastic film component may attempt to manipulate to ensure that the film is suitable for use its intended use. For example, manufacturers may attempt to increase or otherwise control the tensile strength of a thermoplastic film. The tensile strength of a thermoplastic film is the maximum stress that a film can withstand while being stretched before it fails. Another strength parameter that manufacturers may want to increase or otherwise control is tear resistance. The tear resistance of a thermoplastic film is the amount of force required to propagate or enlarge a tear that has already been created in a film. Still further, a manufacturer may want to increase or otherwise control a film's impact resistance.
One way manufacturers may attempt to control or change the material properties of a thermoplastic film is by stretching the film. Common directions of stretching include “machine direction” or “MD” and “transverse direction” or “TD” stretching. As used herein, the term “machine direction” refers to the direction along the length of the film, or in other words, the direction of the film as the film is formed during extrusion and/or coating. As used herein, the term “transverse direction” refers to the direction across the film or perpendicular to the machine direction.
Common ways of stretching film in the machine direction include machine direction orientation (“MDO”) and incremental stretching. MDO involves stretching the film between pairs of smooth rollers. Commonly MDO involves running a film through the nips of sequential pairs of smooth rollers. The first pair of rollers rotates at a speed less than that of the second pair of rollers. The difference in speed of rotation of the pairs of rollers can cause the film between the pairs of rollers to stretch. The ratio of the roller speeds will roughly determine the amount that the film is stretched. For example, if the first pair of rollers is rotating at 100 feet per minute (“fpm”) and the second pair of rollers is rotating at 500 fpm, the rollers will stretch the film to roughly five times its original length. MDO stretches the film continuously in the machine direction and is often used to create an oriented film.
Incremental stretching of thermoplastic film, on the other hand, typically involves running the film between grooved or toothed rollers. The grooves or teeth on the rollers intermesh and stretch the film as the film passes between the rollers. Incremental stretching can stretch a film in many small increments that are evenly spaced across the film. The depth at which the intermeshing teeth engage can control the degree of stretching. Often, incremental stretching of films is referred to as ring rolling.
One common use of thermoplastic films is as food wraps. There are many physical properties that desirable in food grade wrap film. For example, it is desirable that the film has a high machine-direction tear resistance so as to avoid tearing or failing when being pulled off of a roll or wrapped about a container or food. At the same time, it is desirable that the transverse-direction tear resistance is low enough to allow the film to be easily torn from the roll, such as when the film is brought in contact with a cutter bar as found in the standard containers used for containing and dispensing flexible food grade wrap film.
Additionally, it is desirable for the film to be flexible so that consumers can easily mold the film around a container or object to be covered or wrapped. In addition to the foregoing, in order to allow the film to seal around the container or object, it is desirable that the film be somewhat tacky. At the same time, however, if the film is too tacky, the film can aggressively stick to itself causing blocking. This can make the film difficult to unwind from a roll.